Effective reduction of undesirable internal combustion engine emissions, such as hydrocarbon, carbon monoxide, and oxides of nitrogen is provided through catalytic treatment of engine exhaust gas. Efficient catalytic treatment may be provided by passing engine exhaust gas through a catalytic converter which becomes catalytically active when heated, such as through heat energy transfer from engine exhaust gas, to light-off temperature, typically about 400 degrees Celsius. It is known to apply supplemental heating to the converter or to the exhaust gas entering the converter to reduce the time to light-off following an engine coldstart. Advances in supplemental heating efficiency provide that light-off may occur rapidly, for example during initial engine cylinder events, such as engine cranking.
Once a converter reaches light-off and catalytic activity commences, reduction of undesirable exhaust gas constituents such as hydrocarbons HC or carbon monoxide CO through oxidation in the converter may be aided by introducing fresh air in the exhaust gas stream upstream of the converter. A sufficient amount of fresh air must be introduced to provide for complete oxidation, yet the amount must be limited to avoid significant exhaust gas cooling, such as to a temperature below light-off. Accordingly, the amount of air introduced into the exhaust gas stream should be controlled in accord with exhaust gas oxidation requirements, which may be responsive to an estimate of engine air/fuel ratio and exhaust gas flow rate.
To ensure rapid light-off, conventional air control would prevent admission of the relatively low temperature fresh air to the exhaust gas stream during an initial operating period such as during and for a period following an engine coldstart. However, with benefit of advances in supplemental converter heating, proper air control during such an initial period can significantly improve vehicle emissions.
For example, during initial engine cylinder events, such as at the start of engine cranking, it is known to enrich engine air/fuel ratio to provide for more reliable engine startup. The fuel quantity may be further enriched by any fuel hangup quantity present in the engine or the fuel delivery system at startup. This rich startup air/fuel ratio will lead to a period of dramatically increased levels of undesirable exhaust gas constituents, especially HC. Despite a catalytically active converter through the described advances in supplemental converter heating, such increased pollutant levels are not properly oxidized in the converter, as conventional control of fresh air to the exhaust gas stream would preclude a presence of sufficient levels of oxygen in the converter to support such oxidization.
Accordingly, and in recognition of advances in supplemental converter heating, it would be desirable to control admission of fresh air to an internal combustion engine exhaust gas stream so as to provide a sufficient presence of oxygen therein to support oxidation of undesirable exhaust gas constituents during initial engine operating periods.